The present invention relates to high intensity discharge lamps having multiple discharge devices, and more particularly to such lamps having improved lumen recovery upon restarting.
High intensity discharge electric lamps (HID lamps) have found a wide range of applications. These lamps are efficient, they can be made in a wide range of wattages and they have long operating lives. One disadvantageous characteristic of HID lamps; however, is their failure to instantly restart after a momentary power interruption.
HID lamps generally include a discharge device comprising a discharge vessel containing a pair of internal spaced discharge electrodes, and a small quantity of vaporizable and ionizable material referred to as the fill material. Typically, the fill material is a sodium mercury amalgam, and it may also contain other materials such as metal halides. In operation, some of the fill material is vaporized and the voltage applied across the discharge electrodes maintain an electrical discharge through the vaporized material, which is partially ionized. The high temperature vaporized and partially ionized material emits visible light.
A disadvantage inherent in HID lamps is their inability to restart immediately after a momentary power interruption. When the fill material within the discharge vessel has been partially vaporized the internal pressure within the discharge vessel increases to greater than one atmosphere. This pressure increase will result in a higher voltage being required in order to initiate a discharge than in the case of the lamp being started at a lower internal pressure. As a consequence, if power is momentarily interrupted the lamp will have to cool somewhat and the internal pressure of the discharge vessel will have to decrease before discharge can be reestablished.
In order to overcome the delay in restarting inherent in HID lamps, such lamps have been made with multiple discharge devices. U.S. Pat. No. 4,287,454 (Feuersanger et al) discloses HID lamps having a pair of discharge devices connected electrically in parallel. When a starting voltage is applied to this lamp one of the discharge devices starts operating, and its internal pressure rises. If lamp power is momentarily interrupted the starting voltage of the previously operating discharge device will now be too high to allow it to instantly restart. The previously inoperative discharge device, however, will not have a substantially elevated internal pressure so that the reapplied power will cause the latter discharged device to start.
The properties of high pressure sodium lamps having two discharge devices are explored in an article by R.M. Kane and N.R. King, "A 400-W Instant Restrike Double Arc Tube HPS Lamp", Lighting Design + Application, Dec. 1986, pages 31-35. The article examines the ability of an HPS lamp having two discharge devices to restart after a momentary power interruption, to promptly recover lumen output and to develop good light distribution. Because of the presence of a second inoperative discharge device, significant shadowing can occur in the light distribution from the lamp. The principal lamp parameter investigated was the spacing between discharge devices, and its affect upon lamp restarting, lumen recovery and light distribution.
The article shows that there are advantages to closely spacing the two discharge vessels within the lamp. The operative discharge vessel can be used to preheat the inoperative one somewhat. Thus, upon restarting the previously inoperative discharge device will take less time to warm up after restarting. On the other hand, the closer the discharge devices the less likely that the lamp will restart after a second power interruption that shortly follows a first interruption. Moreover, shadowing of the operative discharge device by the inoperative discharge device becomes more severe the closer the discharge devices are spaced.
It would be desirable to provide for some preheating of the inoperative discharge device to improve lumen recovery upon restarting, but without having to space the discharge devices so close as to diminish the restart characteristics or create an intolerable degree of shadowing.